Various types of photosensitive resin compositions are used today in, for example, printing plates, screens for textile printing, and photoresists for fabricating electronic components, or as vehicles in paints, adhesives and printing inks.
For example, when forming a pattern for a printing plate, a negative film or positive film corresponding to a pattern is placed on a photosensitive resin composition layer formed as a film, and light such as ultraviolet rays is irradiated from above the film, thereby exposing portions of the photosensitive resin composition layer. This is followed by development utilizing a difference in solubility with respect to a developer based on a change in chemical structure between exposed areas and unexposed areas, yielding the desired pattern. In order to form a precise and accurate pattern, basic conditions such as a high exposure sensitivity and good developability and resolution are required of the photosensitive resin composition used for such applications and of the photosensitive film obtained by forming this photosensitive resin composition into a film. In addition, storage stability for ensuring the stability of performance is also required. Moreover, in addition to toughness and dimensional stability for ensuring stable printing, the photosensitive film that has been exposed and developed (printing plate) is also required to have durability (solvent resistance, water resistance) to the printing inks, printing plate wash fluids, moisture in air, etc. that are encountered in the course of printing operations and storage.
Emulsified compositions obtained by dispersing a hydrophobic polymer or a mixture of a photopolymerization initiator with a monomer in an aqueous solution containing a water-soluble photocrosslinking agent such as a diazo resin and a polyvinyl alcohol, etc. are known as photosensitive resin compositions for producing such printing plates, and especially screen printing plates.
For example, Japanese Patent Application Laid-open No. S53-51004 (referred to below as “Patent Document 1”) discloses a photosensitive resin composition that includes a polyvinyl alcohol having an average degree of polymerization of at least 1,100 and a degree of saponification of from 75 to 95 mol %, a polyvinyl acetate emulsion or a vinyl acetate-ethylenically unsaturated monomer copolymer emulsion, and a photocrosslinking agent having two or more diazo groups on the molecule (diazo resin).
Japanese Examined Patent Publication No. H4-19542 (referred to below as “Patent Document 2”) discloses a photosensitive resin composition obtained by the emulsification, in an aqueous solution of a partially saponified vinyl acetate polymer having a degree of saponification of from 70 to 95 mol % which contains a water-soluble photocrosslinking agent such as a diazo resin, of a photopolymerization initiator-containing compound that is water-insoluble or sparingly water-soluble and has one, two or more ethylenically unsaturated groups.
In addition, Japanese Examined Patent Publication No. H4-19543 (referred to below as “Patent Document 3”) discloses a photosensitive resin composition obtained by the dispersion, in an aqueous solution of a partially saponified vinyl acetate polymer having a degree of saponification of from 70 to 99 mol % which contains a water-soluble photocrosslinking agent such as a diazo resin or of a partially saponified vinyl acetate polymer having a degree of saponification of from 70 to 99 mol % which contains photocrosslinking groups, of a water-insoluble or sparingly water-soluble, photoactive compound having one, two or more ethylenically unsaturated groups which includes hydrophobic polymer particles and a photopolymerization initiator.
The water-soluble crosslinking agent, i.e., a diazo resin, exerts the greatest influence on the performance of these photosensitive resin compositions. The general synthesis of diazo resins is described on p. 111 of Kankō-sei kōbunshi [Photosensitive polymers] by M. Nagamatsu and H. Inui, pp. 109 to 115, published in 1977 by Kodansha, Ltd. (referred to below as “Non-Patent Document 1”), and in German Patent No. 576731 assigned to Kalle A. G. (referred to below as “Patent Document 4”).
However, the photosensitive resin compositions described in Patent Documents 1 to 3, when stored for one week or more, readily change over time. Particularly in the summertime, they sometimes gel in several days, depending on the condition in which they are stored. Hence, as indicated in column 7 of Patent Document 2 and in column 12 of Patent Document 3, the photocrosslinking agent such as a diazo resin is added immediately before use.
Also, the diazo resins that have hitherto generally been used as water-soluble photocrosslinking agents are diazo resins obtained by condensing a 4-diazodiphenylamine water-soluble salt and paraformaldehyde in sulfuric acid or phosphoric acid (in the diazo resin production examples in Patent Documents 2 and 3, condensation is carried out in sulfuric acid). However, as mentioned above, photosensitive resin compositions that use a diazo resin condensed in sulfuric acid have a poor stability over time. Although photosensitive resin compositions which use a diazo resin condensed in phosphoric acid are relatively stable, they have a low exposure sensitivity; only photosensitive films having a narrow exposure latitude are obtained from such compositions. Another drawback is the inadequate solvent resistance of the photosensitive film following exposure and development.
Moreover, Patent Document 4 and Non-Patent Document 1 merely show the ordinary synthesis of diazo resins. Substantially no investigations have hitherto been conducted on diazo resins capable of forming photosensitive films having both excellent stability over time and a wide exposure latitude.